Radar has long been employed in applications such as air traffic control, fire control, navigation, etc. Due to the many advantages of radar usage in such applications, radar has also been the subject of continuous improvement efforts. As such, for example, digital technology has been developed for use in connection with radar displays. However, certain aspects of older radar technology, such as a long persistence phosphor display to show trails of objects detected via radar, may be desirable for continued use. In this regard, for example, the persistence of the phosphor display enabled operators to see trail data indicative of an object's motion relative to the radar antenna providing the origin for return data. Accordingly, digital video processing techniques have been developed to emulate the long persistence phosphor display. In this regard, previous techniques introduced scan converter hardware for the processing of radar data and the production of a radar image displayed to the radar operator. The scan converter hardware converted analog signals representing radar return data into a final displayable image by first converting analog signals to digital values representing the distance and bearing of a return. A state table was then utilized to store video buildup and trail information based on the digital values. The state table information was then processed for fading video buildup and decaying trails. The state table was then mapped into colors to represent visual trails and video buildup and the color information was written to a video card for display to the operator.
The scan converter described above proved useful in providing many desirable radar display characteristics such as enabling trails and video buildup to be displayed representative of historical return data over a period of time. However, the scan converter described above also included several limitations. In this regard, one limitation of the above described scan converter was that the displayed video image could not be reoriented without losing video buildup and trails. For example, if the radar display was to be shifted from a head up mode, in which the heading of the ship employing the radar is oriented to the top of the display, to a North up mode, in which true North is oriented to the top of the display, then the state table, which recorded data oriented in the same manner as the data was to be displayed, would be cleared and video buildup and trail history data would be lost. Accordingly, for example, it would not be possible to maintain trail history for stabilized head up operation, which may require continuous reorientation of the image based on changes in the heading of the ship.
Another limitation of a prior art scan converter is that repositioning of a video image to support RM(T) operation yields distracting artifacts. RM(T) operation provides for display of relative motion and true trails. Relative motion refers to a mode of operation in which the video origin remains stationary on the display and return data from surrounding objects (e.g., buoys, other ships, coast lines, etc.) move on the display relative to the stationary video origin. True trails refers to a mode of operation in which the trail left behind by target returns will grow at a rate proportional to the object's (or target's) true speed over ground. When attempting to generate true trails, digitized video return data is stored in a state table having a mapping of state table coordinates to real world ground plane coordinates that is constant. However, as a ship moves over the ground plane, the coordinates of the video origin in the state table must move correspondingly. In order to maintain the video origin stationary on the display in relative motion mode, a continuous shift in the mapping between the state table and the display is required. Due to the limited bandwidth of the bus providing platform communication, there cannot be an instantaneous transfer of all the data for adjustment of mapping to accommodate the shift. Thus, only a portion of the data is transferred in a pseudo random fashion so that the display updates gradually rather than instantaneously. The gradual update causes an undesirable pixel twinkle effect that represents a distracting artifact to the user.
An inability to support true motion with relative trails mode (TM(R)) and an inability to provide instantaneous video image offset are additional shortcomings of prior art scan converters. Accordingly, it may be desirable to provide enhanced radar video processing that may overcome at least some of the disadvantages described above.